为什么填分词形式?

这是 现在分词做定语的用法
现在分词放在名词的前面修饰名词, 相当于一个定语从句语法功能.
Anyone trying to take knives on board flights would be caught by the police.
相当于
Anyone who is trying to take knives on board flights would be caught by the police.
再如
The man speaking to the teacher is Tom's father
相当于
The man who is speaking to the teacher is Tom's father.不好意思，括号内填found那就是现在分词作宾补的用法： 现在分词在see, hear, catch, find,keep ,have 等之后可以做宾语的补语： 例如, I see him passing my house every day. I found him stealing things in that shop.